U.S. patent number 11,073,765 [Application Number 16/053,130] was granted by the patent office on 2021-07-27 for method for producing a reflective optical element and reflective optical element.
This patent grant is currently assigned to CARL ZEISS SMT GMBH. The grantee listed for this patent is Carl Zeiss SMT GmbH. Invention is credited to Hartmut Enkisch.
United States Patent |
11,073,765 |
Enkisch |
July 27, 2021 |
**Please see images for:
( Certificate of Correction ) ** |
Method for producing a reflective optical element and reflective
optical element
Abstract
For increasing reflectivity a reflective optical element for the
extreme ultraviolet wavelength range consists of at least two upper
units, in which each upper unit (B1-B5) has a plurality of lower
units, for example reflective optical elements in the form of
mirror arrays. A method for producing the reflective optical
element includes: determination of incidence angles and incidence
angle bandwidths occurring during operation above the surface of
each upper unit (B1-B5); and application of a reflective coating to
each upper unit (B1-B5), adapted to the incidence angles and
incidence angle bandwidths respectively determined above the
surface of each upper unit. This is particularly suitable for
producing reflective optical elements embodied as field facet
mirrors, particularly in the form of microelectromechanical mirror
arrays, for an EUV lithography device.
Inventors: |
Enkisch; Hartmut (Aalen,
DE) |
Applicant: |
Name |
City |
State |
Country |
Type |
Carl Zeiss SMT GmbH |
Oberkochen |
N/A |
DE |
|
|
Assignee: |
CARL ZEISS SMT GMBH
(Oberkochen, DE)
|
Family
ID: |
57944419 |
Appl.
No.: |
16/053,130 |
Filed: |
August 2, 2018 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20180341179 A1 |
Nov 29, 2018 |
|
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
PCT/EP2017/051961 |
Jan 30, 2017 |
|
|
|
|
Foreign Application Priority Data
|
|
|
|
|
Feb 2, 2016 [DE] |
|
|
102016201564.8 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G03F
7/70075 (20130101); G03F 7/7015 (20130101); G03F
7/702 (20130101); G02B 5/0816 (20130101); G03F
7/70116 (20130101); G02B 5/09 (20130101); G02B
5/0891 (20130101) |
Current International
Class: |
G03F
7/20 (20060101); G02B 5/08 (20060101); G02B
5/09 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
10 2004 060 184 |
|
Jul 2006 |
|
DE |
|
10 2011 005 144 |
|
Sep 2011 |
|
DE |
|
10 2010 041 502 |
|
Mar 2012 |
|
DE |
|
10 2011 080 636 |
|
Mar 2012 |
|
DE |
|
10 2011 077 234 |
|
Dec 2012 |
|
DE |
|
10 2011 080 052 |
|
Jan 2013 |
|
DE |
|
10 2012 213 937 |
|
May 2013 |
|
DE |
|
102012213937 |
|
May 2013 |
|
DE |
|
10 2012 205 615 |
|
Oct 2013 |
|
DE |
|
10 2012 212 757 |
|
Jan 2014 |
|
DE |
|
10 2013 203 364 |
|
Sep 2014 |
|
DE |
|
2319951 |
|
May 2011 |
|
EP |
|
2013/014182 |
|
Jan 2013 |
|
WO |
|
2014012660 |
|
Jan 2014 |
|
WO |
|
Other References
Translation of DE-102012213937-A1 (Year: 2013). cited by examiner
.
International Search Report and Written Opinion in counterpart
International Application No. PCT/EP2017/051961, dated Jun. 4,
2017, 6 pages. cited by applicant .
Office Action in corresponding German Application 102016201564.8,
dated Sep. 29, 2018., along with English Translation. cited by
applicant .
International Preliminary Report on Patentability,
PCT/EP2017/051961, date of issuance of this report Aug. 7, 2018, 8
pages. cited by applicant.
|
Primary Examiner: Collins; Darryl J
Assistant Examiner: Lee; Matthew Y
Attorney, Agent or Firm: Edell, Shapiro & Finnan,
LLC
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This is a Continuation of International Application
PCT/EP2017/051961, which has an international filing date of Jan.
30, 2017, and which claims the priority of the German Patent
Application No. 102016201564.8, filed Feb. 2, 2016. The disclosures
of both applications are incorporated in their respective
entireties into the present application by reference.
Claims
What is claimed is:
1. A method for producing a reflective optical element, which is
composed of at least two main units, for the extreme ultraviolet
wavelength range, wherein each main unit has a multiplicity of
sub-units, comprising: determining operational angles of incidence
and angle of incidence bandwidths occurring over respective
surfaces of each main unit; applying a respective reflective
coating on each main unit, wherein each of the respective coatings
is adapted to the operational angles of incidence and angle of
incidence bandwidths which are respectively determined over the
respective surfaces; determining respective maximum bandwidths of
the angles of incidence for each main unit; and applying a
respective multilayer system, having a layer sequence and/or layer
thicknesses that is/are adapted to the overall largest determined
maximum bandwidth, onto each main unit as a reflective coating.
2. A method for producing a reflective optical element, which is
composed of at least two main units, for the extreme ultraviolet
wavelength range, wherein each main unit has a multiplicity of
sub-units, comprising: determining operational angles of incidence
and angle of incidence bandwidths occurring over respective
surfaces of each main unit; applying a respective reflective
coating on each main unit, wherein each of the respective coatings
is adapted to the operational angles of incidence and angle of
incidence bandwidths which are respectively determined over the
respective surfaces; determining maximum bandwidths of the angles
of incidence for each main unit, and then dividing the main units
into bandwidth classes; and applying a respective multilayer
system, having a layer sequence and/or layer thicknesses that
is/are adapted to the maximum bandwidth that is determined for a
respective bandwidth class, onto each main unit as a reflective
coating.
3. A method for producing a reflective optical element, which is
composed of at least two main units, for the extreme ultraviolet
wavelength range, wherein each main unit has a multiplicity of
sub-units, comprising: determining operational angles of incidence
and angle of incidence bandwidths occurring over respective
surfaces of each main unit; applying a respective reflective
coating on each main unit, wherein each of the respective coatings
is adapted to the operational angles of incidence and angle of
incidence bandwidths which are respectively determined over the
respective surfaces; dividing each main unit into surface units;
determining, for each main unit, the angles of incidence occurring
during operation over all the surface units; ascertaining a desired
period length of a multilayer system in dependence on the desired
angles of incidence; approximating variations in the desired period
length over the surface units for each main unit by an nth-degree
polynomial, with n being a non-negative integer; and applying the
corresponding multilayer system to each main unit as a reflective
coating.
4. The method as claimed in claim 3, wherein the surface units
correspond to the sub-units.
5. A method for producing a reflective optical element, which is
composed of at least two main units, for the extreme ultraviolet
wavelength range, wherein each main unit has a multiplicity of
sub-units, comprising: determining operational angles of incidence
and angle of incidence bandwidths occurring over respective
surfaces of each main unit; applying a respective reflective
coating on each main unit, wherein each of the respective coatings
is adapted to the operational angles of incidence and angle of
incidence bandwidths which are respectively determined over the
respective surfaces; wherein applying respective reflective
coatings onto the main units is by locating the main units on a
coating holder, which rotates about an axis, and wherein regions of
constant layer thicknesses are arranged concentrically around the
axis.
6. A reflective optical element for the extreme ultraviolet
wavelength range, composed of at least two main units, wherein each
main unit has a multiplicity of sub-units, wherein each main unit
has a respective reflective coating that is adapted to operational
angles of incidence and angle of incidence bandwidths occurring
over respective surfaces of each main unit, wherein at least one of
the respective coatings is adapted for two or more operational
angles of incidence and two or more angle of incidence bandwidths,
and wherein at least one of said two or more angle of incidence
bandwidths is larger than 2.degree., and wherein the respective
reflective coating of said each main unit is adapted to a largest
maximum bandwidth obtained based on determined respective maximum
bandwidths of the angles of incidence for said each main unit.
7. The reflective optical element as claimed in claim 6, wherein
the reflective coatings are embodied as multilayer systems, having
layer thicknesses and/or a layer sequence which is/are adapted to
the angles of incidence and angle of incidence bandwidths
respectively occurring over the surface of the main unit during
operation.
8. The reflective optical element as claimed in claim 6, wherein
the reflective coatings are embodied as multilayer systems, the
layer thicknesses of which vary with a function that corresponds in
each case to an nth-degree polynomial, with n corresponding to a
non-negative integer.
9. The reflective optical element as claimed in claim 6, wherein
each sub-unit is embodied as an individually actuable mirror.
10. The reflective optical element as claimed in claim 6, embodied
as a field facet mirror.
11. An optical system, having a reflective optical element as
claimed in claim 6.
12. The optical system as claimed in claim 11, configured for
extreme ultraviolet lithography.
13. An EUV lithography apparatus having a reflective optical
element as claimed in claim 6.
14. A reflective optical element for the extreme ultraviolet
wavelength range, composed of at least two main units, wherein each
main unit has a multiplicity of sub-units, wherein each main unit
has a respective reflective coating that is adapted to operational
angles of incidence and angle of incidence bandwidths occurring
over respective surfaces of each main unit, wherein at least one of
the respective coatings is adapted for two or more operational
angles of incidence and two or more angle of incidence bandwidths,
wherein at least one of said two or more angle of incidence
bandwidths is larger than 2.degree., and wherein the respective
reflective coating of said each main unit is adapted to a
respective bandwidth class, wherein the at least two main units are
divided into at least two bandwidth classes based on determined
maximum bandwidths of the angles of incidence for said each main
unit.
Description
FIELD OF THE INVENTION
The present invention relates to a method for producing a
reflective optical element, composed of at least two main units,
for the extreme ultraviolet wavelength range (approximately 1 nm to
20 nm, hereinafter EUV), wherein each main unit has a multiplicity
of sub-units, and to a reflective optical element for the extreme
ultraviolet wavelength range, composed of at least two main units,
wherein each main unit has a multiplicity of sub-units. It
furthermore relates to an optical element and to an EUV lithography
apparatus having such a reflective optical element.
BACKGROUND
DE 10 2012 213 937 A1 discloses a mirror array for use in the
illumination system of an EUV lithography apparatus. This mirror
array is composed of a plurality of sub-arrays, which for their
part have a multiplicity of individual mirrors. In order to allow
simplified maintenance of the illumination optical unit, provision
is made for all individual mirrors of all sub-arrays to be provided
with a reflective coating, which is broad-banded such that it
covers all angles of incidence that may occur during operation of
the mirror array at the various sub-arrays. In particular, this is
a micro-mirror array, which is embodied in the form of
microelectromechanical systems.
SUMMARY
It is an object of the present invention to improve mirror arrays
in particular for EUV lithography such that their reflectivity is
increased.
In a first aspect, this object is achieved by a method for
producing a reflective optical element, which is composed of at
least two main units, for the extreme ultraviolet wavelength range,
wherein each main unit has a multiplicity of sub-units, having the
steps of: determining angles of incidence and angle of incidence
bandwidths occurring over the surface of each main unit during
operation; applying a reflective coating on each main unit, which
is adapted to the angles of incidence and angle of incidence
bandwidths which are respectively determined over the surface
thereof.
This production method has the advantage that by adapting the
reflective coating on each main unit to the angles of incidence and
angle of incidence bandwidths occurring there during operation, the
reflectivity can be increased. Meanwhile, the outlay during the
production of the reflective optical element and later also the
maintenance thereof remains relatively limited, because the
optimization to the angle of incidence bandwidths can in particular
be limited to the main units. In particular in EUV lithography
apparatuses, a plurality of reflective optical elements are
arranged in series. The total reflectivity within EUV lithography
apparatuses is therefore not very high, and any reflectivity yields
are an advantage.
In preferred embodiments, the maximum bandwidth of the angles of
incidence is determined for each main unit, and a multilayer
system, having a layer sequence and/or layer thicknesses that
is/are adapted to the overall largest determined maximum bandwidth,
is applied onto all main units as a reflective coating. This
procedure has the advantage that the outlay during production of
the main units can be kept low.
The multilayer systems in this embodiment, and in the following
embodiments, are preferably multilayer systems based on layers,
arranged in alternation on a substrate, of a material having a
lower real part of the refractive index in the extreme ultraviolet
wavelength range and of a material having a higher real part of the
refractive index in the extreme ultraviolet wavelength range, in
particular at the operating wavelength at which the lithographic
process is performed. The layers, which are arranged in
alternation, can be combined to periods of a specific length. Said
multilayer systems are particularly suitable for the extreme
ultraviolet wavelength range and can be formed, as is known, with
great flexibility due to the selection of the materials, the layer
sequence, and the thickness ratios for desired average angles of
incidence and angle of incidence bandwidths at a selected
wavelength. In a specific layer sequence, it is in particular
possible by changing the layer thicknesses by a constant factor for
the angle of incidence at which the highest reflectivity at a
selected wavelength is achieved to be shifted.
In a further preferred embodiment, the maximum bandwidth of the
angles of incidence is determined for each main unit, and then the
main units are divided into broadband classes, and a multilayer
system, having a layer sequence and/or layer thicknesses that
is/are adapted to the maximum bandwidth that is determined for the
respective bandwidth class, is applied onto each main unit as a
reflective coating. A higher reflectivity of the reflective optical
element can be achieved hereby. This procedure is particularly
preferred in the case of more than two main units. A basic
multilayer system can be prescribed for each class. It is possible
for example to exert influence on the angle bandwidth of a
multilayer system by way of layer sequences that have
aperiodicities or by way of layer thickness gradients perpendicular
to the substrate. Advantageously, the respective basic multilayer
system is designed for the bandwidth of the main unit having the
greatest angle of incidence bandwidth within a class.
Each main unit is particularly preferably divided into surface
units, and for each main unit the angles of incidence occurring
during operation over all the surface units thereof are determined.
Next, a desired period length of a multilayer system is ascertained
in dependence on the desired angles of incidence, and the variation
of the desired period length over the surface units for each main
unit is approximated by an nth-degree polynomial, with n being a
non-negative integer. Subsequently, the corresponding multilayer
system is applied to each main unit as a reflective coating.
In the simplest variants, for each main unit a constant desired
period length is ascertained, which corresponds to a zero-degree
polynomial. Approaches for ascertaining period lengths, which are
desired in dependence on specific angles of incidence occurring
during operation are well-known. For example, DE 2013 203 364 A1
discloses different methods for ascertaining a desired period
length from a maximum and a minimum angle of incidence. It is
possible in this way, among others, in principle to produce the
reflective coatings for all main units in a single coating batch,
wherein different layer thicknesses can be set for the individual
main unit to be coated by way of a period length profile via the
coating holder in the coating system.
In further variants, the profile of the desired period lengths over
the respective main unit can be approximated by first-degree
polynomials, that is to say linear functions. Main units, which
have been designed in this way can also be combined to batches and
be coated in one process. The higher the degree of the polynomial
with which the profile of the desired period length is
approximated, the lower is the deviation of the applied period
length profile from the ideal period length profile. Thereby, the
reflectivity of the main units and of the reflective optical
element which is composed thereof increases. A sufficient thickness
control over the surface can be achieved during coating for example
by the use of honeycomb masks. In the case of more complex
thickness profiles, it is also possible to use a coating method as
described in DE 10 2012 205 615 A1, in which layer-forming
particles are ionized and applied onto the surface to be coated in
a targeted fashion by way of electrical and/or magnetic fields.
Alternatively or additionally, the sub-units can be oriented
differently with respect to the coating source, in order to
influence the applied layer thicknesses in this way as well. With
this embodiment, reflective optical elements can be produced, which
have a particularly high total reflectivity.
It should be pointed out that the profile of the desired period
length can be approximated one-dimensionally in one direction in
the surface or two-dimensionally over the surface of the main
units.
It is particularly advantageous if the surface units correspond to
the sub-units of the main units. To this end, a surface unit can be
identical to the surface of a sub-unit. Depending on the number of
sub-units, it may also make sense for a plurality of neighbouring
sub-units to be combined to surface units, in particular if the
distribution of the angles of incidence or the angle of incidence
bandwidth over these sub-units is comparable.
Advantageously, a reflective coating is applied onto the main units
by locating the main units on a coating holder, which rotates about
an axis, wherein regions of constant layer thicknesses are arranged
concentrically around the axis. Depending on the required layer
thicknesses or period lengths, the main units can be arranged at
different distances from the axis of rotation of the coating holder
in order to be able to coat main units with reflective coatings,
which are adapted differently to angles of incidence and angle of
incidence bandwidth occurring during operation, in common
batches.
In a further aspect, the object is achieved by a reflective optical
element for the extreme ultraviolet wavelength range, composed of
at least two main units, wherein each main unit has a multiplicity
of sub-units, in which each main unit has a reflective coating that
is adapted to the angles of incidence and angle of incidence
bandwidths respectively occurring over the surface of the main unit
during operation. Such composed reflective optical elements have a
higher reflectivity than those known from the prior art, yet are
producible with an outlay, which is not too great.
The reflective coatings are preferably embodied as multilayer
systems, having layer thicknesses and/or a layer sequence which
is/are adapted to the angles of incidence and angle of incidence
bandwidths respectively occurring over the surface of the main unit
during operation. In particular, each main unit of the reflective
optical element has a multilayer system, which is adapted to the
angles of incidence occurring during operation specifically in this
main unit. With respect to the angle of incidence bandwidth, all
main units, or in each case at least two main units, can have
multilayer systems which are adapted to the same angle of incidence
bandwidth. In particular, the multilayer systems can have
aperiodicities or thickness gradients perpendicular to the
multilayer system surface.
In preferred embodiments, the reflective coatings are embodied as
multilayer systems, the layer thicknesses of which vary with a
function that corresponds in each case to an nth-degree polynomial,
with n being a non-negative integer. This can be a one-dimensional
or a two-dimensional polynomial over the surface of the multilayer
system of zeroth, first, second, third, fourth or any higher
degree.
Preferably, each sub-unit is implemented as an individually
actuable mirror. With very particular preference, each main unit is
implemented as a microelectronic system of micro-mirrors.
Consequently, the reflective optical element can be used
particularly well in optical systems or in EUV lithography
apparatuses in places where elements with a relatively great
surface area are required, which must be aligned differently and
precisely at the same time over the entire surface.
The reflective optical element is preferably implemented as a field
facet mirror. In particular, reflective optical elements based on
microelectromechanical systems of micro-mirrors, such as mirror
arrays, are suitable to be used as field facet mirrors. The tilt of
the individual actuable micro-mirrors is set during operation such
that both the tilt of a field facet and the curvature thereof are
emulated. Here, each main unit contributes, for example as a
sub-array, to a plurality of field facets, and each field facet is
made up of sections of a plurality of main units. Each field facet
has an individual average angle of incidence and an individual
bandwidth of angles of incidence. The reflective optical element
described here is particularly suitable for offering good
reflectivity by taking into account the distribution of the angles
of incidence over the field facet mirror. The reflective optical
element introduced here can also be implemented as a pupil facet
mirror.
In further aspects, the object is achieved by an optical system, in
particular for EUV lithography, or by an EUV lithography apparatus
having a reflective optical element as described above.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be explained in more detail with
reference to preferred exemplary embodiments. In the Figures:
FIG. 1 shows a schematic view of an embodiment of an EUV
lithography apparatus;
FIG. 2 shows a schematic view of an embodiment of an illumination
system,
FIG. 3 shows a schematic view of an embodiment of a reflective
optical element with seven main units;
FIG. 4 shows a schematic view of a main unit;
FIG. 5 shows a schematic of a multilayer system;
FIG. 6 shows the distribution of angles of incidence over the
surface of a reflective optical element having five main units;
FIG. 7 shows the reflective coating of a conventional reflective
optical element with five main units;
FIG. 8 shows the reflective coating of a first embodiment of a
reflective optical element according to the invention with five
main units;
FIG. 9 shows the reflectivity as a function of angles of incidence
for two different multilayer systems;
FIG. 10 shows the reflective coating of a second embodiment of a
reflective optical element according to the invention with five
main units;
FIGS. 11, 12 show two further schematic views of the reflective
optical element from FIG. 3;
FIG. 13 shows one possible arrangement of main units to be coated
in a coating machine;
FIG. 14 shows the reflective coating of a third embodiment of a
reflective optical element according to the invention with five
main units;
FIG. 15 shows the reflective coating of a fourth embodiment of a
reflective optical element according to the invention with five
main units;
FIG. 16 shows the average reflectivity for average angles of
incidence for the optical reflective elements from FIGS. 10 to
14;
FIG. 17 shows the average reflectivity for extreme angles of
incidence for the optical reflective elements from FIGS. 10 to
14;
FIG. 18 shows the average PV value for average angles of incidence
for the optical reflective elements from FIGS. 10 to 14; and
FIG. 19 shows the average PV value for extreme angles of incidence
for the optical reflective elements from FIGS. 10 to 14.
DETAILED DESCRIPTION
FIG. 1 shows a schematic view of a projection exposure apparatus
100 for producing, for example, microelectronic devices, which is
operated in a scan mode along a scan direction 126 at an operating
wavelength in the EUV range and which can have one or more optical
elements with additional coating. The projection exposure apparatus
100 shown in FIG. 1 has a point-type plasma radiation source. The
radiation from the laser source 102 is directed, via a condenser
104, onto suitable material, which is introduced via the feed 108
and excited to a plasma 106. The radiation emitted by the plasma
106 is imaged by the collector mirror 110 onto the intermediate
focus Z. Appropriate stops 111 at the intermediate focus Z ensure
that no undesired stray radiation is incident on the subsequent
mirrors 112, 114, 116, 118, 120 of the illumination system of the
projection exposure apparatus 100. The plane mirror 122 serves for
folding the system, so as to offer installation space for
mechanical and electronic components in the object plane in which
the mount for the reticle 124 is arranged. The mirror 112 in the
present example is followed in the illumination system by a field
facet mirror 114 and a pupil facet mirror 116. The field facet
mirror 114 serves to project a multiplicity of images of the
radiation source of the projection exposure apparatus into a pupil
plane, in which a second facet mirror is arranged, which serves as
the pupil facet mirror 116 and superposes the images of the facets
of the field facet mirror 114 in the object plane so as to make
possible the most homogeneous lighting possible. The mirrors 118
and 120, which are arranged downstream of the facet mirrors 114,
116, substantially serve to form the field in the object plane.
Arranged in the object plane is a structured reticle 124, the
structure of which is imaged onto the object 130 to be exposed, for
example a wafer, using a projection lens 128, which has six mirrors
in the present example. The reticle 124 in the projection exposure
apparatus 100, which is designed here as a scanning system, is
displaceable in the marked direction 126 and is successively lit in
sections in order to correspondingly project the respective
structures of the reticle 124 onto, for example, a wafer 130 using
the projection lens.
FIG. 2 shows a radiation source in connection with an illumination
system 11, which is part of a projection exposure apparatus for EUV
lithography. A collector 1 is arranged around a light source that
is formed by a plasma droplet 2, which is excited by an infrared
laser 3. In order to obtain wavelengths in the region around, for
example, 13.5 nm in the EUV wavelength range, e.g. tin can be
excited to a plasma using a carbon dioxide laser operating at a
wavelength of 10.6 .mu.m. Instead of a carbon dioxide laser, it is
also possible to use solid-state lasers, for example. The collector
1 is followed, downstream of the stop 5 at the intermediate focus
4, by a field facet mirror 16 having individual facets 18 and a
pupil facet mirror 17 having individual facets 19. Before the rays
8 are incident on the reticle 13, which has the structure that is
to be projected onto a wafer and which is to be scanned in the
y-direction, they are deflected by a folding mirror 12. The folding
mirror 12 has less of an optical function and rather serves to
optimize the space requirement of the illumination system 11.
It should be noted that a wide variety of radiation sources can be
used in UV or EUV lithography, including plasma sources which may
be based on laser excitation (LPP sources) or gas discharge (DPP
sources), synchrotron radiation sources and free electron lasers
(FEL). Furthermore, the collectors can have any desired design,
including as a Wolter collector or as an ellipsoidal collector,
preferably adapted to the radiation source that is used in each
case.
The facet mirrors, in particular the field facet mirror, in the
present example are embodied as a reflective optical element for
the extreme ultraviolet wavelength range, composed of at least two
main units, wherein each main unit has a multiplicity of sub-units,
and wherein each main unit has a reflective coating that is adapted
to the angles of incidence and angle of incidence bandwidths
respectively occurring over the surface of the main unit during
operation. The field facet mirror was produced by first determining
the angles of incidence and angle of incidence bandwidths occurring
over the surface of each main unit during operation and
subsequently applying a reflective coating on each main unit, which
is adapted to the angles of incidence and angle of incidence
bandwidths which are respectively determined over the surface
thereof.
FIG. 3 shows a reflective optical element 30 according to the
invention having seven main units 31, which is designed to replace
a conventional field facet mirror 40 having seven field facets 41,
which is not implemented as a micro-mirror array. As is shown by
way of example in FIG. 4, one main unit 31 has a multiplicity of
sub-units 32, which, in the present example, are implemented in the
form of individually actuable micro-mirrors. Together they form a
microelectromechanical system.
As shown in FIG. 3, each field facet 41 is formed by a plurality of
main units 31, and each main unit 31 makes a contribution to more
than one field facet 41. Each field facet 41 has an individual
angle of incidence and an individual angle of incidence bandwidth
owing to the various positions and curvature of the former.
Consequently, different angles of incidence with different angle of
incidence bandwidths are also incident at different locations of
each main unit 31.
FIG. 5 schematically shows the structure of a sub-unit 50 of the
reflective optical element. The illustrated example shows a
micro-mirror element based on a multilayer system 51, which has
been applied on a substrate 52. Materials having a low coefficient
of thermal expansion are preferably chosen as substrate materials.
The multilayer system 51 substantially comprises alternately
applied layers of a material with a higher real part of the
refractive index at the operating wavelength at which for example
the lithographic exposure is carried out (also called spacer 55)
and of a material with a lower real part of the refractive index at
the operating wavelength (also called absorber 54), wherein in the
example shown here, an absorber-spacer pair forms a stack 53 which
corresponds to a period in the case of periodic multilayer systems.
In certain respects a crystal is thereby simulated whose lattice
planes correspond to the absorber layers at which Bragg reflection
takes place. The thicknesses of the individual layers 54, 55 and
also of the repeating stacks 53 can be constant over the entire
multilayer system 51 or vary, depending on what spectral or
angle-dependent reflection profile is intended to be achieved. The
reflection profile can also be influenced in a targeted manner by
the basic structure composed of absorber 54 and spacer 55 being
supplemented by further more and less absorbent materials in order
to increase the possible maximum reflectivity at the respective
operating wavelength. To this end, absorber and/or spacer materials
can be interchanged in some stacks, or additional layers of other
materials may be provided. The absorber and spacer materials can
have constant or varying thicknesses over all the stacks in order
to optimize the reflectivity. Furthermore, it is also possible to
provide in individual or all stacks additional layers for example
as diffusion barriers between spacer and absorber layers 55, 54 to
increase the thermal stability. The first layer adjoining the
substrate 52 can be an absorber layer, a spacer layer or an
additional layer. To protect the reflective coating against
external influences, it is possible for a protective layer 56,
which can also include more than one layer, to be provided as a
termination with respect to the vacuum. Preferred materials for the
EUV wavelength range include molybdenum as the absorber material
and silicon as the spacer material.
Broadband reflective coatings can be produced in typical ways by
way of multilayer systems, which are, for example, periodic having
a low period number, have two or more periodic sub-systems with
different period lengths, or are completely aperiodic. In addition,
they can have a layer thickness gradient in the direction
perpendicular to the substrate. The angle of incidence with the
highest reflectivity for a specific multilayer system can be
shifted, for example, by varying the period length for said
multilayer system. Various approaches as to how a desired period
length for the multilayer system of a reflective optical element is
determined as reflective coating for example from an angle of
incidence onto said reflective optical element which is maximum
during operation and one which is minimum during operation is
described, for example, in DE 2013 203 364 A1. A disadvantage when
selecting an identical reflective coating for all main units is the
relatively low total reflectivity.
FIG. 6 shows, by way of example, the angles of incidence over the
surface of a reflective optical element, which is embodied as a
micro-mirror array. By way of example, it has five main units B1 to
B5, which are embodied as sub-arrays and have a multiplicity of
actuable micro-mirrors as sub-units. In the present example, the
reflective optical element serves as a facet mirror, in particular
as a field facet mirror, wherein each main unit in section-wise
fashion corresponds to one of five conventional field facets. In
these sections, during operation in, for example, the illumination
system of an EUV lithography apparatus, different angle of
incidence distributions are incident on each surface unit of a main
unit B1 to B5, which can respectively be assigned to a field facet.
Of these are plotted for each main unit B1 to B2 in FIG. 6, in
degrees over various positions which are assigned to in each case
one surface unit or one field facet, the maximum angle of incidence
as AOI_max (short dashes), the average (mean) angle of incidence as
AOI_mn (solid line), and the minimum angle of incidence as AOI_min
(long dashes). The respective angle of incidence bandwidth can be
ascertained via the difference between minimum and maximum angle of
incidence.
In the example shown in FIG. 6, the maximum angles of incidence in
the first main unit B1 vary between approx. 7.5.degree. and approx.
15.5.degree., the minimum angles of incidence between approx.
4.5.degree. and approx. 11.5.degree., and the angle of incidence
bandwidth between approx. 2.degree. and approx. 7.degree.. In the
second main unit B2, the maximum angles of incidence vary between
approx. 11.degree. and approx. 18.degree., the minimum angles of
incidence between approx. 4.degree. and approx. 12.5.degree., and
the angle of incidence bandwidth between approx. 2.degree. and
approx. 8.degree.. In the third main unit B3, the maximum angles of
incidence vary between approx. 8.5.degree. and approx. 18.degree.,
the minimum angles of incidence between approx. 3.5.degree. and
approx. 15.5.degree., and the angle of incidence bandwidth between
approx. 2.degree. and approx. 7.5.degree.. In the fourth main unit
B4, the maximum angles of incidence vary between approx.
5.5.degree. and approx. 17.5.degree., the minimum angles of
incidence between approx. 3.5.degree. and approx. 13.degree., and
the angle of incidence bandwidth between approx. 2.degree. and
approx. 8.degree.. In the fifth main unit B5, the maximum angles of
incidence vary between approx. 9.5.degree. and approx.
16.5.degree., the minimum angles of incidence between approx.
5.degree. and approx. 13.degree., and the angle of incidence
bandwidth between approx. 2.degree. and approx. 5.degree..
FIG. 7 shows a reflective optical element, composed of five main
units B1 to B5, according to the prior art, which was mentioned in
the introductory part. On all five main units B1 to B5, it has an
identical reflective coating, which is designated with "const."
Additionally shown is the ideal desired period length over the
individual positions, which is designated with "ideal." The
reflective coating in the present example is a periodic multilayer
system. To optimize the reflectivity, the period length was
extended globally over all main units B1 to B5 by a little more
than 3%.
To increase the total reflectivity of the reflective optical
element, in particular when used as a field facet mirror in the
illumination system of an EUV lithography apparatus, it is proposed
to take account of the angles of incidence and the angle of
incidence distribution occurring during operation in the reflective
coating of the main units separately for each main unit. The
reflective coatings are advantageously embodied as multilayer
systems, having layer thicknesses and/or a layer sequence which
is/are adapted to the angles of incidence and angle of incidence
bandwidths respectively occurring over the surface of the main unit
during operation. Advantage is taken here in particular of the fact
that the angle of incidence with the highest reflectivity at a
specific wavelength can be changed by varying the period length,
especially when using multilayer systems as the basis of the
reflective coating which is suitable for a specific angle of
incidence bandwidth.
In a first exemplary embodiment, the reflective coating of the
reflective optical element is embodied as a multilayer system of a
specific layer sequence, which is based on a broadbandedness that
is desired for all main units. In this embodiment, the desired
broadbandedness preferably takes its cue from the main unit having
the greatest occurring angle of incidence bandwidth. The desired
period length is optimized for each main unit individually in
dependence on the angles of incidence occurring during operation.
In this way, the variation of the desired period length over the
surface units for each main unit is approximated by a zero-degree
polynomial, and a corresponding reflective coating is applied.
FIG. 8 shows such an embodiment which, analogously to the
illustration in FIG. 7, likewise has five main units B1 to B5 and
is designed for use as a field facet mirror with the angles of
incidence shown in FIG. 6. For each main unit B1 to B5, the desired
period length has been determined for each surface unit in
dependence on the angles of incidence occurring there. In
accordance with the angles of incidence and angle of incidence
distribution ascertained for each main unit B1 to B5, a calculation
was performed as to the factor by which the desired period length
of the multilayer system, on which the reflective coating is based,
with desired broadbandedness for each main unit should be modified.
In the example shown in FIG. 8, the factor varies between just
under 1.03 to almost 1.04. The period length for the respective
main unit is drawn as a thick solid line, which is designated with
"mean."
Since the reflective coating for all main units B1 to B5 is based
on the same multilayer system with desired broadbandedness, and
consequently both the materials, the sequence thereof in the form
of layers, and the layer thickness ratios correspond for all main
units, all five main units can be coated in one batch, wherein a
different layer thickness distribution is set via the coating
holder. Depending on which main unit is intended to have which
period length, they are arranged on the coating holder
appropriately for the coating. For example, a coating holder, which
rotates about an axis can be used herefor. Regions of constant
layer thicknesses are situated on concentric circles around the
axis of rotation.
Depending on the ascertained angle of incidence bandwidth, the main
units can be divided into different classes. In the example
illustrated here, two classes can be used. The main units B1 and
B5, which have an angle of incidence bandwidth of approx.
12.degree., are placed in the first class. The main units B2, B3
and B4, which have an angle of incidence bandwidth of approx.
14.degree., are placed in the second class (see FIG. 6). The basic
multilayer systems are selected differently for both classes,
adapted to the required bandwidth. FIG. 9 shows the reflectivity in
dependence on the angle of incidence at a wavelength of 13.5 nm for
both basic multilayer systems in the form of a dashed line,
designated with "standard," for the first class, and as a solid
line, designated with "broadband," for the second class. The two
classes can be coated in two different batches. It is also possible
to obtain coatings with different broadbandedness on different
radii of the coating holder in the case of a variation of a
rotation-symmetrical thickness profile of the individual layers
such that a thickness gradient perpendicular to the substrate is
obtained. Additionally, the respectively desired period length is
also taken into account, as explained, in the case of the coating
of each individual main unit.
In a further embodiment illustrated schematically in FIG. 10, the
reflective coatings of the main units B1 to B5 in the reflective
optical element are implemented as multilayer systems, the layer
thicknesses of which correspond to a first-degree polynomial. To
produce this embodiment, each main unit is divided into surface
units, and for each main unit the angles of incidence occurring
during operation over all the surface units thereof are determined.
Next, ascertained in dependence on the desired angles of incidence
is a desired period length of a multilayer system, which
approximates the variation of the desired period length over the
surface units for each main unit by a first-degree polynomial.
Subsequently, the corresponding multilayer system is applied to
each main unit as a reflective coating.
In the example illustrated here in FIG. 10, again each main unit B1
to B5 is divided into in each case five surface units, specifically
as in the other exemplary embodiments explained here, in a manner
such that each surface unit belongs to a different field facet and
combines sub-units, for example individual micro-mirrors, having
similar angles of incidence and angle of incidence bandwidths. They
are designated with a position from 1 to 25. The resulting period
length for each main unit B1 to B5, which is approximated by a
straight line, is shown in FIG. 10 as a thick solid line, which is
designated with "grad."
The approximation of the variation of the average angle of
incidence by linear gradients in as few directions as possible
permits the coating of all main units in one batch. To this end, a
layer thickness profile is set during the coating, which oscillates
with a short spatial wavelength via the coating holder. FIG. 11
schematically supplements the illustration from FIG. 3 by the
variation of the period length, which is indicated by hatching.
FIG. 12 omits the field facets so as to better show the hatching.
Thicker lines indicate a higher period length than thinner lines.
Required in region A is a linear gradient from maximum to minimum
to maximum period length over the diagonal of the main unit,
required in region B, likewise over the diagonal, is a linear
gradient from minimum to maximum to minimum period length, required
in region C, likewise over the diagonal, is a linear gradient from
maximum to minimum period length, and required in region D, in the
longitudinal direction, is a linear gradient from minimum to
maximum period length. In a coating machine, in which layer
thicknesses can be applied in a radial-geometrically oscillating
fashion, the main units to be coated can be arranged accordingly on
the coating holder, depending on the desired gradient for the
period length, as is schematically illustrated in FIG. 13, wherein
the circles which are concentric around the spin axis S of the
coating holder are dashed lines of constant layer thickness.
In the example shown in FIG. 14, each main unit B1 to B5 was
divided as for previous examples. However, the variation of the
ideal desired period length over the surface units for each main
unit was approximated by a polynomial of a higher degree. The
resulting relative period length of the basic multilayer system for
each main unit B1 to B5 is shown in FIG. 13 as a thick solid line,
which is designated with "spline."
This approximation of the variation of the average angles of
incidence for each main unit is preferably performed not only in a
direction longitudinally with respect to a linear gradient, but
two-dimensionally over the entire surface of the respective main
unit. A corresponding reflective coating with two-dimensional local
thickness variations can be produced, for example, using honeycomb
masks. For more complex thickness distributions, a temporally
controllable method without a mask can be used, as disclosed for
example in DE 10 2012 205 615 A1.
The more coefficients of a higher degree are taken into account,
the better can be the approximation of the profile of the ideal
desired period length. FIG. 15 schematically illustrates an example
of a profile of the period length, which coincides with the profile
of the ideal period length. This profile is drawn as a thick solid
line and designated with "ideal." Such a reflective coating can be
produced easily in particular if the sub-units of each main unit
are individually actuable micro-mirrors, such as in the context of
microelectromechanical systems. Since the applied coating thickness
also depends on the orientation of the surface to be coated with
respect to the particle source, each micro-mirror can be tilted for
the coating such that the actually deposited thickness corresponds
to the desired thickness. In particular, the unevennesses in the
ideal desired period length over the surface of the main unit can
be approximated thereby.
The effect of the procedure proposed here will be illustrated on
the basis of the following FIGS. 16 to 19. FIG. 16 shows the
average reflectivity for the entire field facet mirror calculated
for the exemplary embodiments introduced here in accordance with
FIGS. 8 ("mean"), 10 ("grad"), 14 ("spline") and 15 ("ideal"). The
reflectivity is here normalized to the value of the conventional
reflective optical element in accordance with FIG. 7. Adapting the
reflective coating to the average angle of incidence over each
entire main unit can already produce a significant reflectivity
gain, the average reflectivity can be increased by more than 5%
with the linear approximation, by approximately 12.5% with the
quadratic approximation, and even by more than 15% by way of
adaptation to the average angle of incidence of each field
facet.
Furthermore examined were also examples in which the main units
have additionally been divided into two broadbandedness classes and
the main units B1 and B5 were provided with the basic multilayer
system "standard" explained in connection with FIG. 9 and the main
units B2 to B4 were provided with the basic multilayer system
"broadband." The corresponding values in FIG. 16 have the addition
"+BB." In particular in the embodiments "mean" and "grad" it is
possible by taking account of the angle of incidence bandwidths to
achieve an additional increase in the total reflectivity by a few
percent.
These increases in reflectivity can be established to an even
greater extent for the average reflectivity in the extreme angles
of incidence, that is to say the minimum and maximum angles of
incidence (see FIG. 17).
Also considered were the average peak-to-valley values for the
average angle of incidence (FIG. 18) and the extreme angles of
incidence (FIG. 19), which are a measure of the deviation of the
actual coating from the ideal reflective coating, which is
optimized at each surface point to the angles of incidence, which
are respectively incident there. The peak-to-valley values are
again normalized to the value for the example 7 according to the
prior art. The procedure explained here can be used to approximate
the ideal coating well in particular for the extreme angles of
incidence.
* * * * *